This invention relates to a photopolymerisation and photocrosslinking process, and in particular to such a process for the production of images.
Conventionally, production of an image by means of photopolymerisation is achieved by coating a support with a solution in a volatile organic solvent of a photopolymerisable substance, causing or allowing the solvent to evaporate so leaving a film of the photopolymerisable substance, irradiating the film with actinic radiation as through an image whereby the parts of the film struck by the irradiation become photopolymerised (and less soluble) while those parts shielded from the irradiation remain substantially unaffected, then dissolving away the unirradiated, unphotopolymerised parts of the film by means of a suitable solvent which does not dissolve the irradiated, photopolymerised parts. This last stage is conventionally known as "development".
It would be desirable to have a process in which a layer of a photopolymerisable substance were applied to a support and this layer were converted into a substantially solid, non-tacky state, ready for irradiation, without the use of organic solvents. Not only would, in this stage, the use be avoided of solvents which might present problems of toxicity and flammability and also cause expense in their recovery, but the production on a continuous basis of coated supports, ready for irradiation, would be facilitated.
We have found that this object can be achieved by the use of certain substances which contain in the same molecule two kinds of groups through which photopolymerisation can occur at rates which differ considerably from one another. The groups are chosen so that photopolymerisation of a layer of a liquid composition occurs rapidly to form a solid, essentially tack-free layer, which is, however, still soluble in certain solvents. When desired, parts of the layer are further subjected to a substantially greater amount of actinic radiation and photocrosslinking takes place through the other type of group in the already photopolymerised molecules of the layer, the parts of the layer which undergo photocrosslinking becoming much more resistant to solution in the solvents.
Brazilian published Patent Application No. 8008428 describes a method for forming relief images from a film of a liquid photopolymerisable material, comprising exposing the film to actinic radiation so that it solidifies by chemical hardening, then re-exposing the solidified film to further actinic radiation in the form of a pattern so that parts of the film become chemically differentiated, and then selectively removing the portions of the film not exposed to the patterned exposure of actinic radiation by washing with a solvent. The only photopolymerisable materials suggested comprise a mixture of a polyene and a polythiol.
Unpublished experiments by the Applicants have shown that numerous potentially useful compounds, containing two types of units normally capable of undergoing photopolymerisation, do not give satisfactory results in such a process, photopolymerisation in the first stage being much retarded, apparently as a consequence of the presence in the molecule of another type of photopolymerisable unit, despite the incorporation of a variety of photoinitiators and photosensitisers. In other unpublished experiments of the Applicants, mixtures of two compounds, one containing a unit normally capable of undergoing photopolymerisation and the other containing a unit normally capable of undergoing dimerisation on exposure to irradiation, gave unsatisfactory results, apparently due to inhibition of the photocrosslinking reaction.
It has been found that the desired process can be achieved by employing a compound which contains in the same molecule both one or more acryloyl or methacryloyl groups and one or more bicyclo[2.2.1]hept-2-ene structures.